This application relates to a system and method for providing light emission from a semiconductor laser and more particularly, for controllably broadening the linewidth of a semiconductor laser.
Optical and opto-electronic devices frequently require light sources with specifically tailored optical properties. For example, an interferometer fiber optic gyroscope (FOG), which detects movement of an object, is an example of such an opto-electronic device. To avoid interference effects arising from multiple reflections in the optical circuit of a FOG, a suitable light source should have a linewidth great enough so that the de-coherence length of the fiber is shorter than the distance between discontinuities in the circuit. For example, in a FOG, the de-coherence length of the fiber should be shorter than the distance between a fiber splice and a directional coupler. The de-coherence length of an optical fiber is determined by the wavelength and the linewidth of the light propagating in the fiber.
Lasers are known to emit light in a narrow emission cone which can advantageously be coupled into fibers with a high coupling efficiency. However, the narrow spectral linewidth and associated large coherence length make laser sources unsuitable for fiber optic gyros. On the other hand, the linewidth below the lasing threshold, i.e., in the spontaneous emission regime, is broad, but not much usable power is generated.
Fiber optic gyros can be operated using a superluminescent diode as a light source. Superluminescent diodes have a light-guiding structure in the form of an optical waveguide and emit light more directionally than LEDs made in the form of a simple p-n junction structure. Superluminescent diodes, however, have become very expensivexe2x80x94as compared to, for example, lasers used in CD disk playersxe2x80x94due to low commercial demand, thereby increasing the overall cost of fiber optic gyros. It is therefore desirable to provide a high-power and low-cost light source which has the advantageous properties of a superluminescent diode.
According to one aspect of the invention, a method and a system are disclosed for controllably broadening the spectral characteristics of a semiconductor laser to produce a superluminescent diode. The method includes charging a transmission line with a predetermined charge, connecting one end of the transmission line to electrical terminals of the semiconductor laser, with the other end of the transmission line forming a reflective circuit, possibly repeatedly discharging the transmission line through the semiconductor laser, and monitoring the spectral characteristics of radiation emitted at a laser facet at a predetermined laser current until the spectral characteristics of the emitted radiation match predetermined spectral characteristics.
According to another aspect of the invention, a method is disclosed for producing a semiconductor light source suitable for use in a fiber-optic sensor having a predetermined birefringence. The method includes providing a semiconductor laser initially having spectral characteristics which produce a de-coherence length greater than a predetermined spacing in the fiber-optic sensor, charging a transmission line with a predetermined charge, connecting one end of the transmission line to electrical terminals of the semiconductor laser, with the other end of the transmission line forming a reflective circuit, possibly repeatedly discharging the transmission line through the semiconductor laser, and monitoring the spectral characteristics of radiation emitted at a laser facet at a predetermined laser current, until the spectral characteristics of the semiconductor laser produce a de-coherence length which is smaller than the predetermined spacing in the fiber-optic sensor.
Embodiments of the invention may include one or more of the following features. The spectral characteristics may be the full-width-at-half-maximum (FWHM) of an emission spectrum of the laser measured at a predetermined laser current which causes lasing of the semiconductor laser before the transmission line is discharged through the semiconductor laser. The transmission line may be a coaxial cable. The predetermined charge may be provided by a power supply connected, possibly via a resistor, to the distal end of the transmission line, i.e., the end opposite to the end where the laser is connected. The resistor may have a sufficiently high resistance to cause a voltage pulse traveling along a longitudinal direction of the transmission line to be reflected at the distal end of the resistor. The length of the coaxial cable may be selected to discharge the coaxial cable through the semiconductor laser by a current pulse having a pulse duration between 10 and 1000 ns, preferably approximately 100 ns.